The present invention relates to an image forming apparatus and an image forming head for use in, e.g., a printer for computers, facsimiles, copying machines and the like, for driving charged particles from a charged-particle supporting element onto an image receiving medium according to an image signal.
In this type of image forming apparatuses, an insulating member having a plurality of openings for passing charged particles therethrough is provided between a supporting element for supporting and carrying the charged particles and an opposing electrode, an image receiving medium is provided between the opposing electrode and the insulating member, and a control electrode is provided around each opening. For image formation, a potential difference is provided in advance between the supporting element and the opposing electrode so as to form an electrostatic field for transferring the charged particles from the supporting element toward the opposing electrode. A voltage applied to the control electrodes is controlled to electrostatically open and close the openings, whereby the charged particles are separated from the supporting element onto the image receiving medium through the openings, according to an image signal.
Japanese Laid-Open Publication No. 58-122882 describes that, upon detecting absence of an image receiving medium, spark discharge is generated between control electrodes and an opposing electrode or the like by application of a high voltage to the control electrodes, so as to blow off the toner stuck in openings.
Japanese Laid-Open Publication No. 58-104769 describes that, when image formation is not conducted, an electric field between control electrodes and an image receiving medium is increased so as to remove the toner stagnant in openings toward the image receiving medium.
Japanese Laid-Open Publication No. 58-104771 describes that, during image recording, an electric field between a supporting element and an image receiving medium as well as an electric field in openings are produced in the traveling direction of charged particles toward the image receiving medium, and during non-recording, respective electric fields between the supporting element and a control member and between the control member and the image receiving medium are produced in the opposite direction to that during recording, whereby clogging of the openings with the toner is prevented.
However, in the aforementioned method for removing the charged particles in the openings by spark discharge, an insulating member having the openings may be damaged by the spark discharge if it is formed from a synthetic resin. Moreover, an additional power supply is required to generate the spark discharge, and the charged particles may possibly be fusion-bonded on the insulating member due to heating by the spark discharge.
Moreover, in the method for removing the charged particles in the openings when no image is formed (during non-recording), the charged particles stuck in the openings cannot be removed, if any, during the image formation, i.e., while the charged particles are being sequentially attached to, e.g., a sheet of recording paper according to an image signal. Moreover, the aforementioned methods all require a special voltage-application mode to remove the charged particles in the openings, and also require a special power supply, thereby often increasing the costs.
In other words, it is an object of the present invention to prevent clogging of the openings without voltage control of the control electrodes.
The present invention solves the aforementioned problems by applying ideas to the area of the opening.
More specifically, according to the present invention, an image forming apparatus for forming an image by attaching charged particles to an image receiving medium includes:
a charging means for applying charges to particles for forming an image;
a supporting element for supporting and carrying the charged particles having the charges applied thereto by the charging means;
an opposing electrode provided at a position facing a position on the supporting element to which the charged particles are carried;
an insulating member provided between the supporting element and the opposing electrode and having a plurality of openings for passing the charged particles therethrough;
a control electrode provided around each opening of the insulating member;
a transfer-electrostatic-field forming means for providing a potential difference so as to form between the supporting element and the opposing electrode a transfer electrostatic field for transferring the charged particles on the supporting element toward the opposing electrode; and
a voltage control means for applying a voltage to the control electrodes around the respective openings according to an image signal so as to control passage of the charged particles through the respective openings that is caused by the transfer electrostatic field, wherein
a percentage of an area of the opening to a sum of an area of an extent of the control electrode around the opening and the area of the opening is 8% or more.
The present invention will be specifically described. The inventor arranged ring-shaped control electrodes surrounding respective circular openings, and applied a pulsed driving voltage to the control electrodes so that the control electrodes each have, at a respective opening position, an intermediate potential of the voltage difference between an opposing electrode and a developing sleeve (supporting element), thereby causing charged particles on the developing sleeve to be intermittently driven onto an image receiving medium through the openings. The inventor then observed the resultant driving trace on the developing sleeve (the trace where the charged particles have gone away). This driving trace did not have a circular shape corresponding to the shape of the opening, but a large circular shape corresponding to the outer shape of the control electrode. In other words, dots are not only formed from the charged particles present on a location of the developing sleeve that corresponds to the opening, but the charged particles present on a location of the developing sleeve that corresponds to the control electrode also contributes to the dot formation. This means that not only the charged particles present on the location of the developing sleeve that corresponds to the opening, but also the charged particles present on the location of the developing sleeve that corresponds to the control electrode are separated away from the developing sleeve toward the opening and pass therethrough, in response to application of the driving voltage to the control electrode.
The reason why the charged particles present on the location of the developing sleeve that corresponds to the control electrode move toward the opening is as follows: in response to application of the driving voltage, the charged particles present in the space right above the opening (the space between the location of the developing sleeve corresponding to the opening and the opening location of the insulating member) are discharged through the opening, whereby the charged-particle concentration in the space is reduced. However, as can be seen from the equipotential lines shown in FIG. 4, an electrostatic field surrounding the control electrode 19 is produced with its potential progressively increased toward the control electrode 19. Accordingly, with the reduction in charged-particle concentration right above the opening 16, the electrostatic field moves the charged particles from a position around the space right above the opening 16 toward the opening 16.
Thus, not only the charged particles on the location of the developing sleeve that corresponds to the opening 16, but also the charged particles on the location of the developing sleeve that corresponds to the control electrode 19 are going to pass through the opening 16. Therefore, when the area of the control electrode 19 is too large as compared to the area of the opening 16, not all of these charged particles smoothly pass through the opening 16, resulting in clogging of the opening 16.
Then, paying attention to the area of the opening 16 and the area of the extent of the control electrode 19 around the opening, the inventor defined that the percentage of the opening area to the sum of the area of the extent of the control electrode around the opening and the opening area is set to 8% or more. Herein, the area of the extent of the control electrode around the opening is the projected area of the control electrode onto the plane orthogonal to the shortest line connecting the supporting element and the opposing electrode through the opening, or the projected area of the control electrode onto the supporting element.
Since the aforementioned percentage is 8% or more in the present invention, the opening can be prevented from being clogged with the charged particles. This will become apparent from the embodiments described below.
Moreover, according to the present invention, an image forming apparatus for forming an image by attaching charged particles to an image receiving medium includes:
a charging means for applying charges to particles for forming an image;
a supporting element for supporting and carrying the charged particles having the charges applied thereto by the charging means;
an opposing electrode provided at a position facing a position on the supporting element to which the charged particles are carried;
an insulating member provided between the supporting element and the opposing electrode and having a plurality of openings for passing the charged particles therethrough;
a control electrode provided around each opening of the insulating member;
a transfer-electrostatic-field forming means for providing a potential difference so as to form between the supporting element and the opposing electrode a transfer electrostatic field for transferring the charged particles on the supporting element toward the opposing electrode; and
a voltage control means for applying a voltage to the control electrodes around the respective openings according to an image signal so as to control passage of the charged particles through the respective openings that is caused by the transfer electrostatic field, wherein
a percentage of an area of the opening to an area of a portion of the supporting element that is affected by the control electrode so as to drive the charged particles into the opening in response to application of the voltage from the voltage control means to the control electrode is 8% or more.
More specifically, as described above, not only the charged particles present on a location of the supporting element that corresponds to the opening, but also the charged particles present on a location of the supporting element that corresponds to the control electrode are driven toward the opening in response to application of a voltage to the control electrode around the opening. However, if the control electrode extends widely around the opening, or a part of the control electrode widely stretches out, not all of the charged particles present on the location of the supporting element that corresponds to the control electrode are driven toward the opening, but only the charged particles on the location of the supporting element that is affected by the control electrode are driven toward the opening.
In other words, when the control electrode extends widely around the opening, the charged particles on a location of the supporting element that corresponds to the peripheral portion of the control electrode are less likely to be driven toward the opening. When a part of the control electrode widely stretches out, the charged particles on a location of the supporting element that corresponds to the stretch-out portion are less likely to be driven toward the opening.
Accordingly, the opening area is preferably determined such that the percentage of the area of the opening to the area of the portion of the supporting element that is affected by the control electrode so as to drive the charged particles into the opening in response to application of the voltage from the voltage control means to the control electrode is 8% or more.
When the supporting element has a cylindrical shape with the charged particles supported on its peripheral surface, the portion of the supporting element that is affected by the control electrode is present within a distance of 50 xcexcm or less from a tangent line passing through a point on the supporting element that corresponds to the center of the opening. More specifically, it is now assumed that the supporting element is curved with a prescribed radius of curvature (e.g., a radius of 15 to 20 mm). When the radius of curvature is large, a location beyond the distance of 50 xcexcm is so far from the opening that it is less likely to be affected by the control electrode. When the radius of curvature is small, an angle between the normal line of a location beyond the distance of 50 xcexcm and the line connecting that location and the opening is so large that the charged particles are less likely to be driven toward the opening. Accordingly, the distance is preferably 50 xcexcm or less.
Moreover, the upper limit of the aforementioned percentage is as close to 100% as possible, if the dot density resulting from the charged particles driven onto the image receiving medium through the opening is not concerned. However, in order to improve the dot density while increasing the opening diameter so as to prevent clogging with the charged particles, the width of the control electrode surrounding the opening must be reduced as much as possible. Thus, the upper limit of the percentage is determined from the minimum possible width of the control electrode to be manufactured. More specifically, the minimum possible width of the control electrode produced by, e.g., a current etching method is about 20 xcexcm. In this case, the upper limit of the percentage is about 52%. A laser processing method can reduce the minimum possible width to about 10 xcexcm, and therefore the upper limit of the percentage is about 70%. Note that, for example, the minimum electrode width corresponds to the width W of the control electrode 19 shown in FIG. 5.
When a ring-shaped electrode surrounding the opening is used as the control electrode, the percentage of the area of the opening to the area enclosed by the outer periphery of the control electrode may be set to 8% or more, and the upper limit thereof may be set to about 52% or about 72%.
The opening area is preferably 900xcfx80 (unit: xcexcm2) or more. This can prevent clogging of the opening with the charged particles. It should be noted that whether or not the opening is likely to be clogged with the charged particles largely depends on the particle size of the charged particles. As the particle size decreases, the lower limit of the opening area is reduced. Note that xcfx80 indicates the ratio of the circumference of a circle to its diameter.
For example, the upper limit of the opening area may be 10,000xcfx80 (unit: xcexcm2)
Moreover, particles having a volume-average diameter of 5 to 15 xcexcm may be used as the charged particles.
According to the present invention, an image forming head provided in front of a supporting element having charged particles for forming an image supported thereon, for controlling driving of the charged particles toward an image receiving medium includes:
an insulating member having a plurality of openings for passing the charged particles therethrough; and
a control electrode provided around each opening of the insulating member, and receiving a voltage for controlling passage of the charged particles through the respective opening, wherein
a percentage of an area of the opening to a sum of the area of the opening and an area of a portion of the control electrode that extends around the opening and causes the charged particles on the supporting element to be driven toward the opening in response to application of the voltage to the control electrode is 8% or more.
More specifically, as described above, when the control electrode extends widely around the opening, the peripheral portion of the control electrode does not act on substantial image formation (does not serve to drive the charged particles on the supporting element toward the opening). In addition, when a part of the control electrode widely stretches out, the stretch-out portion does not act on substantial image formation.
Accordingly, in the case of forming the image forming head, the area of the opening may be determined based on the sum of the area of the opening and the area of the portion of the control electrode that extends around the opening and causes the charged particles on the supporting element to be driven toward the opening.
In the case of such an image forming head as well, the control electrode preferably has a ring shape surrounding the respective opening. For example, the upper limit of the percentage of the area of the opening may be set to 70%. The area of the opening may be set in the range of 900xcfx80 to 10,000xcfx80 (unit: xcexcm2). Particles having a volume-average diameter of 5 to 15 xcexcm may be used as the charged particles.
As has been described above, according to the present invention, the percentage of the area of the opening to the sum of the area of the extent of the control electrode around the opening and the area of the opening is 8% or more, or the percentage of the area of the opening to the area of the portion of the supporting element that is affected by the control electrode so as to drive the charged particles into the opening is 8% or more, or the percentage of the area of the opening to the sum of the area of the opening and the area of the portion of the control electrode that extends around the opening and causes the charged particles on the supporting element to be driven toward the opening is 8% or more. Therefore, the openings can be prevented from being clogged during image formation without applying to the control electrodes a special voltage for preventing clogging of the openings. This is advantageous to prevention of damages to components of the image forming apparatus, cost reduction, and reliable image formation (dot formation).